1. Field of the Invention
The present invention relates to a thermoprotector in which the melting point or the softening point of a fusible material is set as the operating temperature, a thermosensor which is useful in the thermoprotector, and a method of producing a thermosensor.
2. Explanation of Related Art
As a thermoprotector which senses abnormal heating of an electrical or electronic apparatus, and which performs a cut-off operation based on this sense to interrupt the apparatus from a power supply, thereby preventing overheat of the apparatus and occurrence of a fire, a system in which elastic distortion energy is stored and the elastic distortion energy is released by melting or softening of a fusible material is known.
For example, an elastic metal piece 2′ is forcibly bent as shown in (10A) of FIG. 10, the both ends of the elastic metal piece 2′ are bonded against a bending reaction force to a pair of stationary terminals 41′, 42′ by an fusible alloy (solder) 3′ having a predetermined melting point. When the ambient temperature is raised to the melting point of the fusible alloy 3′ and the fusible alloy is melted, bending stress of the elastic metal piece 2′ is released to cancel the joining between one end of the elastic metal piece 2′ and the one stationary terminal 42′ as shown in (10B) of FIG. 10, thereby interrupting the power supply (see Japanese Patent Application Laying-Open No. 7-29481).
As shown in (11A) of FIG. 11, a device is known in which a pellet 2′ having a predetermined melting point, a seat plate 15′, a compression spring 1′, and a seat plate 16′ are sequentially housed in a metal case 14′ to which a lead terminal 13′ is attached at one end, with starting from the one end. Furthermore, a contact 42′ in which the outer circumference is in sliding contact with the inner face of the metal case is housed in the case, a lead pin bushing 17′ is fixed to the other end side of the metal case 14′, and a trip spring 18′ is incorporated between the bushing 17′ and the contact 42′, thereby constituting a conduction path passing the route of the lead terminal 13′→the metal case 14′→the contact 42′→a lead pin 41′. When the ambient temperature is raised to the melting point of the pellet 2′ and the pellet 2′ is melted, compression stress of the compression spring 1′ is released, and the contact 42′ is detached from the tip end of the lead pin 41′ by compression stress of the trip spring 18′ as shown in (11B) of FIG. 11, thereby interrupting the conduction path (see “ELECTRICAL ENGINEERING HANDBOOK” First Edition, The Institute of Electrical Engineers of Japan, Feb. 28, 1988, p. 818).
In the system shown in FIG. 10, however, the bending reaction force M′ and an expanding force F′ of the elastic metal piece act on the fusible alloy (solder). Therefore, the stress distribution in the fusible alloy is complicated, creep due to stress concentration is readily produced, and an operation failure easily occurs. Since the fusible alloy forms a part of a conduction path, the fusible alloy may generate heat because of an increase of the resistance due to creep of the fusible alloy, thereby causing a possibility that an operation error may be caused by self-heating. Furthermore, an operation error may be caused also by stringing of the molten alloy.
In the system shown in FIG. 11, the pellet can be uniformly compressed by pressure equalization of the seat plates, but the structure is complicated. Therefore, the system is inevitably disadvantageous in miniaturization and cost.